The invention relates generally to the field of marine wellbore drilling from multiple wellbore, water bottom supported platforms. More specifically, the invention relates to devices for re-using conductor pipe in depleted wellbores drilled from such platforms so that additional wellbores may be drilled from the same platform, wherein the platform has a limited number of “slots” for location of pipe in the upper portion of such wellbores.
Wellbores are drilled from bottom supported platforms in bodies of water such as lakes or the sea for, among other purposes, extracting hydrocarbons from formations below the bottom of the body of water. Such platforms typically includes “slots” in one of more horizontally disposed frame like structures for the location of the portion of the wellbore that extends from the water bottom to a part of the platform above the water surface. The wellbores are typically drilled with selected trajectories to access selected formations at subsurface geodetic locations displaced from the geodetic location of the platform.
When all platform slots are full, that is, when wellbores are drilled such that the upper portions thereof fill all the slots, it is generally necessary to wait for one or more of the wellbores to no longer be usable (e.g., when production of hydrocarbons stops or no longer takes place at an economically useful rate), at which point such wellbore(s) may be plugged and abandoned. The associated slot(s) may then be used to drill “infill” wellbore(s). Such wellbores typically have trajectories selected to access additional hydrocarbon bearing formations or previously unproduced portions of such formations penetrated by other wellbores at different depths and/or geodetic locations. Such procedure enables reuse of slots repeatedly.
The manner in which the reuse of slots is performed depends on many factors, e.g., water depth, the number of conductor pipe guides on the platform and the distance between them, the ability of platform to withstand new additional loads later in its life (i.e., fatigue strength), and how the reclaimed well was originally constructed.
As an example, the assignee of the present invention operates a platform in 49 meters depth of water with one platform guide located 8.8 meters below sea level and having 12 slots. The wellbores on this platform typically have short producing lifetimes and the well slots are frequently recycled/reclaimed for new “infill” wells. The wells on the foregoing described platform have been constructed in a variety of ways during its operating lifetime, but wellbores with mud line suspension systems, and wellbores that were predrilled and then tied back after the platform was installed present the most difficult mechanical problems during the slot reclamation process.
It has been the practice of the assignee of the present invention to reclaim the slots up to the base of the structural conductor pipe of the wellbore in each reclaimed slot, enabling reuse of the conductor pipe and enabling redrilling wellbores from the depth of the conductor pipe (typically about 50 meters below the water bottom). There have been numerous problems with the foregoing type of slot reclamation, ranging from complications during cleaning out the wellbore's mud line suspension systems and tieback systems (the foregoing devices are located just below the water bottom or “mud line”) thus making it difficult to successfully drill replacement (infill) wells along different selected trajectories to establish the new infill wells. A typical problem in drilling infill wells from slots reclaimed in such manner is exposure to formations that are known to have low fluid pressures, thus resulting in fluid loss during drilling.
The assignee of the present invention has built and used “mud line” (water bottom) whipstocks that are set on cut casing strings, but these are typically set on wells where there is no seabed template. Such whipstocks are typically 20 inches diameter and are located below the conductor pipe, again with the intention of reusing the conductor pipe in a reclaimed wellbore. Such whipstocks have been primarily designed to manage the difficulties associated with achieving successful cement side-track plugs in loss situations.
Dealing with the issue of working around a large seabed template has also been evaluated. Some companies provide a type of “ramp” of that sits in the cut casing string. Then a new conductor string is run and is deflected over and around the seabed template via the ramp. This concept is typically associated with conductors that are driven into the water bottom sediments. It is desirable to drill a conductor pipe hole prior to running the conductor pipe due to the nature of the sediments in place.